[gdb/symtab] Handle DW_AT_string_length with location list

Consider a fortran routine where a string variable s is modified:
...
subroutine f(s)
  character*(*) s
  print *, s
  s(1:3) = 'oof'
  print *, s
end subroutine f
...

When compiling with optimization level -O1 and printing the type of
variable s we get:
...
$ gdb -q -batch outputs/gdb.opt/fortran-string/fortran-string \
  -ex "b f" \
  -ex run \
  -ex "ptype s"
Breakpoint 1 at 0x4006f7: file fortran-string.f90, line 21.

Breakpoint 1, f (s=..., _s=_s@entry=3) at fortran-string.f90:21
21      subroutine f(s)
type = character*1
...
while with -O0 we have instead:
...
type = character (3)
...

The problem is that the type of s is:
...
 <1><2d6>: Abbrev Number: 21 (DW_TAG_string_type)
    <2d7>   DW_AT_string_length: 0xbf (location list)
    <2db>   DW_AT_byte_size   : 4
...
where the DW_AT_string_length is a location list, a case that is not handled
by attr_to_dynamic_prop.

Fix this by handling attr->form_is_section_offset () in attr_to_dynamic_prop.

Tested on x86_64-linux.

The test-case is based on gdb.opt/fortran-string.exp from
https://src.fedoraproject.org/rpms/gdb/raw/f32/f/gdb-archer-vla-tests.patch .
I've updated the copyrights to stretch to 2021.

[ I've tried to create a dwarf assembly test-case for this, but didn't
manage. ]

Co-Authored-By: Jan Kratochvil <jan.kratochvil@redhat.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26910

gdb/dwarf2/read.c

@@ -18468,16 +18468,32 @@ attr_to_dynamic_prop (const struct attribute *attr, struct die_info *die,
     }
   else if (attr->form_is_constant ())
     prop->set_const_val (attr->constant_value (0));
-  else
+  else if (attr->form_is_section_offset ())
     {
+      switch (attr->name)
+	{
+	case DW_AT_string_length:
+	  baton = XOBNEW (obstack, struct dwarf2_property_baton);
+	  baton->property_type = default_type;
+	  fill_in_loclist_baton (cu, &baton->loclist, attr);
+	  prop->set_loclist (baton);
+	  gdb_assert (prop->baton () != NULL);
+	  break;
+	default:
+	  goto invalid;
+	}
+    }
+  else
+    goto invalid;
+
+  return 1;
+
+ invalid:
   dwarf2_invalid_attrib_class_complaint (dwarf_form_name (attr->form),
 					 dwarf2_name (die, cu));
   return 0;
 }
 
-  return 1;
-}
-
 /* See read.h.  */
 
 /* Read the DW_AT_type attribute for a sub-range.  If this attribute is not

gdb/testsuite/gdb.opt/fortran-string.exp

@@ -0,0 +1,38 @@
+# Copyright 2009-2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/> .
+
+# Test GDB can cope with Fortran strings having their length present in a CPU
+# register.  With -O0 the string length is passed on the stack.  To make this
+# test meaningful the follow assertion should pass.  It is not being checked
+# here as the "_s" symbol is compiler dependent:
+#   (gdb) info address _s
+#   Symbol "_s" is a variable in register XX.
+
+standard_testfile .f90
+
+if { [prepare_for_testing "prepare for testing" ${testfile} ${srcfile} \
+	  {debug f90 additional_flags=-O1}] } {
+    return -1
+}
+
+if ![runto f] then {
+    perror "couldn't run to f"
+    continue
+}
+
+gdb_test_no_output "set print frame-arguments all"
+gdb_test "frame" ".*s='foo'.*"
+gdb_test "ptype s" "type = character \\(3\\)"
+gdb_test "p s" "\\$\[0-9\]* = 'foo'"

gdb/testsuite/gdb.opt/fortran-string.f90

@@ -0,0 +1,27 @@
+! Copyright 2009-2021 Free Software Foundation, Inc.
+!
+! This program is free software; you can redistribute it and/or modify
+! it under the terms of the GNU General Public License as published by
+! the Free Software Foundation; either version 3 of the License, or
+! (at your option) any later version.
+!
+! This program is distributed in the hope that it will be useful,
+! but WITHOUT ANY WARRANTY; without even the implied warranty of
+! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+! GNU General Public License for more details.
+!
+! You should have received a copy of the GNU General Public License
+! along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+subroutine f(s)
+  character*(*) s
+  print *, s
+  s(1:3) = 'oof'
+  print *, s
+end subroutine f
+
+program main
+  character*3 s;
+  s = 'foo'
+  call f (s)
+end program main